A combustion engine comprises one or more pistons for driving a crank shaft. The crank shaft converts the linear, e.g., up and down, movement of each piston into a rotational movement in which the crank shaft rotates about a crank shaft axis. Each piston has an end section situated inside a cylinder. The cylinder and the respective piston together form a combustion chamber. Fuel and air may be ignited in the combustion chamber to exert a force on the piston, thereby driving the piston. The mixture of air and fuel inside the combustion chamber needs to be ignited at an appropriate moment in each drive cycle of the piston. That is, the air fuel mixture in the cylinder should be ignited when the piston is at a certain position relative to the cylinder. The air fuel mixture may be ignited by generating a spark inside the combustion chamber. The spark may be generated for example by interrupting an electrical current through an induction coil. More specifically, it may be desirable to ignite the air fuel mixture when the piston is at a predefined fixed position, referred to herein as the ignition position, relative to its top dead centre (TDC) position. TDC is the position in which the volume of the combustion chamber is minimal. The TDC is one of the two turning points of the piston. Depending on the design of the engine, the optimal instant for triggering the spark may be shortly before or after the piston is at TDC In other words, the ignition position may be near the TDC. The instantaneous position or phase of the piston or, equivalently, of the crank shaft, may be determined, for example, using a trigger wheel.
An example of an engine 10 comprising a cylinder (not shown), a piston 12, and a crank shaft 16 is schematically shown in FIG. 1. The piston 12 is located inside the cylinder and delimits with the cylinder a combustion chamber, this being the volume above the piston and within the closed end of the cylinder. The piston 12 is capable of linear, e.g., up and down, movement when a mixture of air and fuel in the combustion chamber of the cylinder is ignited at suitable times, e.g., each time the piston 12 is approximately at its top dead centre. The piston 12, possibly in conjunction with one or more other pistons (not shown), is connected to the crank shaft 16 via the moveable conrod 14, and thus drives the crank shaft 16 to rotate about the crank shaft axis 18. One cycle of the piston 12, that is, the time it takes the piston 12 to complete one cycle of motion, e.g., from top dead centre to top dead centre, may translate into one revolution of the crank shaft.
The engine 10 may further comprise a trigger wheel 20 connected to the crank shaft 16 and which is rotatable with the crank shaft 16 about the crank shaft axis 18. The trigger wheel 20 may be connected rigidly to the crank shaft 16, or they may be formed in one piece. Accordingly, one revolution of the crank shaft 16 may result in one corresponding revolution of the trigger wheel 20. In another example (not shown) a trigger wheel comparable to the trigger wheel 20 may be connected to the crank shaft via a gear assembly. In this case, the trigger wheel may have a rotational cycle shorter or longer than the rotational cycle of the crank shaft. The trigger wheel 20 may have a circumference 22 which may be dented. For example, the circumference 22 of the trigger wheel 20 may exhibit an alternating series of teeth 24 and recessions 26.
The engine 10 may further comprise a trigger wheel sensor 28. The trigger wheel sensor may be arranged near the trigger wheel 20 so as to generate a trigger wheel signal in response to rotation of the trigger wheel 20. In the example, the trigger wheel sensor 28 comprises an induction sensor 30 configured to induce an electrical voltage in response to a magnetic flux which may be modulated by the motion of the trigger wheel 20. In the example, the trigger wheel sensor 30 comprises a core 32 comprising a ferromagnetic material. The trigger wheel 20 or the core 32 or both may be at least partly magnetic or a permanent magnet may be operably coupled to the core 32. A gap 34 between the core 32 and the trigger wheel 20 may be wider or narrower in dependence of the rotational position of the trigger wheel. More specifically, the gap 34 may be narrow when one of the teeth 24 is facing the core 32 and wider when one of the recessions 26 is facing the core 32. The trigger wheel sensor 30 may further comprise a coil 36. The coil 36 may have one or more loops around the core 32. An electrical voltage may thus be induced in the coil in accordance with the rotational motion of the trigger wheel 20. The coil 36 may have a differential output 38, 40 for providing the induced voltage.
In the example, the trigger wheel sensor 28 further comprises a comparator 42 having a differential input connected to the differential output 38, 40 of the coil 36. The comparator 42 may be incorporated into the sensor or implemented remotely in an electronic control unit, for example. The comparator 42 may be configured to produce an output potential in response to the voltage received from the differential output 38, 40 of the coil 36. The trigger wheel sensor 28 may thus generate a trigger wheel signal, e.g., the output potential from the comparator 42, in response to rotation of the trigger wheel 20. The comparator 42 may introduce a certain delay between the trigger wheel signal and the induced voltage at the differential output 38, 40. The trigger wheel signal may be fed to an ignition controller (e.g., the micro-controller unit 54 in FIG. 4) driving an ignition device (not shown) located near or inside the combustion chamber in the cylinder to ignite the air fuel mixture in the combustion chamber at times adapted to the position of the trigger wheel 20 and thus adapted to the motion of the piston 12. The ignition controller may be set so as to achieve an optimal timing of the ignition moment relative to the position of the piston 12.
FIG. 2 schematically shows the trigger wheel 20 and the induction sensor 30 from FIG. 1. Typically, the voltage induced by, e.g., the coil 36 in response to rotation of the trigger wheel 20, more specifically the passing of a tooth edge, has a certain phase lag relative to the trigger wheel's tooth edge passing the induction sensor. For example, the phase lag of the induced voltage should ideally be zero and the peak voltage should occur when the tooth edge is passing the induction sensor. As a consequence of the many geometrical tolerances and imperfections of the components between and including the trigger wheel 20 and the piston 12, the induced voltage from the coil 36 may have a phase lag relative to a detected tooth and thereby a phase lag relative to top dead centre position of the piston 12 which differs noticeably from the ideal value of zero. These combined errors may all contribute to a non-ideal phase lag between the actual position of the trigger wheel when the piston is at TDC and the indicated position of TDC from the trigger wheel itself, as illustrated schematically by the two sinusoidal graphs in the figure, wherein the plain graph refers to an ideal signal and the crossed graph refers to an observed signal from the induction sensor 30.
In other words, the accuracy of the trigger wheel angle as measured by the trigger wheel sensor 28 may be affected by manufacturing and other tolerances. Such tolerances may be the result of mechanical, electrical and magnetic effects. Mechanical tolerances may include, for example, tolerances of the trigger wheel teeth machining, the alignment of the trigger wheel relative to the crank shaft, the placement of bored holes, tolerances of the alignment of the induction sensor 30 relative to the trigger wheel 20, and clearances between bolts and bored holes. Electrical tolerances may for example include tolerances of a phase shift or delay of the trigger wheel sensor 28, as well as tolerances of the circuitry connected to the induction sensor 30.
The phase shift between the observed sensor signal (plain graph in FIG. 2) and the ideal signal (crossed graph in FIG. 2) may be the accumulated effect of various manufacturing tolerances. The phase shift may have a negative impact on the performance of the engine. Notably, the phase shift may cause the ignition spark to be triggered too early or too late. In engines in which fuel, e.g., gasoline, is injected directly into the cylinder at a suitable moment in each combustion cycle of the cylinder, both the timing of the spark and timing of the fuel injection may suffer from an imperfect phase shift of the output signal of the trigger wheel sensor. This phase shift is equivalent to an imperfect offset of the trigger wheel angles represented by the output signal of the trigger wheel sensor 28 and illustrated in FIG. 2. The most sensitive engines in this respect are, in order, engines using in-cylinder pressure control, diesel engines, and gasoline direct injection engines.
For example, combustion-generated pressure before TDC may slow the engine, whereas combustion-generated pressure after TDC may speed it up. The engine may therefore be quite sensitive to the timing of the ignition relative to the TDC, i.e., relative to the instant at which the piston is at the TDC. For example, FIG. 3 shows the pressure inside the combustion chamber, i.e., the in-cylinder pressure as a function of the crank angle in different scenarios. Graph A illustrates the in-cylinder pressure as a function of the crank angle in a scenario in which the piston is driven, e.g., by another engine, and no ignition is triggered, in order to show the pure pressure profile that may result from a variation of the volume of the combustion chamber alone. Graph B shows the in-cylinder pressure as a function of the crank angle in a scenario in which the ignition is retarded and the mechanical energy extracted is poor. Graph C refers to a scenario in which the ignition is timed correctly and thus although the combustion energy is the same as with Graph B, significantly greater mechanical energy is extracted. Graph D shows the in-cylinder pressure in a scenario in which the fuel air mixture is ignited too early, causing knock and thus potentially causing engine damage. Therefore the accurate knowledge of crank angle and timing of ignition is central to the efficient operation of an internal combustion engine.
In particular, some calculations of engine performance such as Indicated Mean Effective Pressure using data from an in-cylinder pressure sensor might be 10% out for an error in crank position of only 1 degree. Errors of even this magnitude might have a significant detrimental impact on the ability to control an engine using Homogeneous Charge Compression Ignition.